Beta-based titanium alloy with low elastic modulus

ABSTRACT

Provided is a beta-based titanium alloy with a low elastic modulus, including no elements harmful to the human body and having excellent biocompatibility. The beta-based titanium alloy includes titanium (Ti), niobium (Nb) and zirconium (Zr) as major alloying elements, and further includes tantalum (Ta), hafnium (Hf), molybdenum (Mo), tin (Sn), and the like. The beta-based titanium alloy has a much lower elastic modulus than the typical biomedical titanium alloys, and thus can resolve the problem of so-called “stress shield effect.” Therefore, the beta-based titanium alloy can be widely used as a material for general civilian goods such as eyewear frames and headsets and sports and leisure goods, as well as a biomedical material for artificial bones, artificial teeth and artificial hip joints.

TECHNICAL FIELD

The present disclosure relates to a titanium alloy with a low elasticmodulus, including no elements harmful to the human body, and moreparticularly, to a beta-based titanium alloy with a low elastic modulus,including titanium (Ti), niobium (Nb) and zirconium (Zr), and furtherincluding tantalum (Ta), hafnium (Hf), molybdenum (Mo), tin (Sn), andthe like.

BACKGROUND ART

Titanium is widely used in the fields of aerospace, weaponry, nuclearpower, sports and leisure, biomedicine and the like due to its highspecific strength (strength/weight), high corrosion resistance,excellent mechanical properties including high temperature properties,and excellent biocompatibility.

Biomedical metals have been developed for use in implants for replacingbones, joints, teeth, and the like. The biomedical metals are used formanufacturing a variety of prostheses such as artificial bones,artificial joints, and dental prostheses. Accordingly, biomedical metalsshould be excellent in biocompatibility as well as mechanicalproperties, corrosion resistance, and chemical resistance. That is,biomedical metals should be non-toxic and not induce allergies in thehuman body.

Titanium and titanium alloys have been used as biomaterial for replacingstainless steel. In the beginning, pure titanium and titanium alloy suchas Ti-6Al-4V were used as biomaterial.

However, since it came to light that aluminum can cause Alzheimer'sdisease and vanadium is cytotoxic in the human body, unceasing effortshave been made to develop a new biocompatible alloy based on titanium.

Widely known examples of biocompatible titanium alloys that have beendeveloped to solve the problem of cytotoxicity are Ti-6Al-7Nb andTi-5Al-2.5Fe, which are second-generation titanium alloys.

Since the 1990s, the problem of “stress shield effect” was newly raised.The stress shield effect is caused by elastic modulus difference betweennatural bone with a low elastic modulus and biocompatible material witha high elastic modulus.

For example, a metal implant with a high elastic modulus bears most ofthe load applied to the region around the implant, and the natural bonein the region does not bear any tension, compression and bending for along time. As a result, the thickness and the weight of the natural boneare reduced gradually, causing serious problems such as osteoporosisaround the implant. This phenomenon is referred to as the “stress shieldeffect.” As the natural bone weakens and the density of the osseoustissue of the cortex decreases, the bonding strength between the naturalbone and the artificial implant also decreases, resulting in decreasedservice life of the implant.

As a result, a demand arose for a biomedical metal that satisfiesbio-mechanical suitability requirements as well as bio-chemicalsuitability requirements including cytotoxicity. That is, a demandemerged for a metal that is not harmful to the human body and has anelastic modulus as low as bones of the human body.

Ti-13Nb-13Zr (ASTM F1713), Ti-12Mo-6Zr-2Fe (ASTM F1813), Ti-15Mo (ASTMF2066), and the like have been developed throughout the world to solvethe above mentioned problems. In addition, a variety of alloys such asTi-35Nb-5Ta-7Zr and Ti-16Nb-13Ta-4Mo in a similar composition range arebeing developed.

However, the titanium alloys hitherto developed have an elastic modulusof approximately 60 GPa to approximately 80 GPa, which is still muchhigher than the elastic modulus of natural bones that range fromapproximately 10 GPa to approximately 30 GPa. Accordingly, the problemof “stress shield effect” has not been completely solved yet. Therefore,there is a considerable demand for a material that is not harmful to thehuman body and, at the same time, has a lower elastic modulus.

Considering that an alloying element of a high melting point may makethe alloying process more difficult and increase the manufacturing cost,the ease of the fabrication process should be taken into account in thedevelopment of a titanium alloy.

DISCLOSURE Technical Problem

The present disclosure provides a titanium alloy composition that is notharmful to the human body, has an elastic modulus as low as bones of thehuman body, and at the same time, is melted and cast easily andcost-effectively.

Technical Solution

Noting that a beta phase generally has a low elastic modulus, theinventors selected alloying elements of titanium alloy on the basis ofwhether they can serve as a beta stabilizer in titanium alloy to lowerthe elastic modulus of titanium alloy.

Also, the inventors selected the alloying elements of titanium alloy onthe basis of whether they are harmless to the human body in terms ofbiochemical suitability, and whether the density, melting temperatureand boiling temperature thereof are economically suitable when comparedto titanium. Resultantly, as beta stabilizers satisfying the aboverequirements, niobium (Nb) and zirconium (Zr) were selected.

Then, the inventors designed a titanium alloy composition having a lowelastic modulus using a semi-experimental method for designing anddeveloping an alloy. The method includes calculating the covalent bondorder and the energy level of the electrons according to the content ofeach alloying element, using the electronic state, which is the core ofthe discrete variational (DV)-Xa molecular orbital method.

Most properties of a material are determined by the electronic state ofthe material except when a nuclear reaction is involved. Based on theelectronic state determining micro-properties of the material on anatomic scale, we can estimate the macro-properties of the material byperforming statistical-mechanical analysis. Here, the micro-propertiesof the material can be analyzed approximately from the electronic stateof the material by interpreting the Schrodinger equation and the like.

The inventors calculated the bonding order, B_(o) and the energy levelof the electrons, M_(d) of the above-described alloying elements throughthe DV-Xa molecular orbital method, and discovered a beta-based titaniumalloy composition with a low elastic modulus from there-among.

In accordance with an exemplary embodiment, the titanium alloy with alow elastic modulus includes from 37 wt. % to 41 wt. % niobium (Nb),from 5 wt. % to 8 wt. % zirconium (Zr), and a balance of titanium, withunavoidable impurities. The titanium alloy has an elastic modulus of 55GPa or lower.

As such, it is possible to realize a low elastic modulus of 55 GPa orlower, which is difficult to realize in a related art Ti—Nb—Zr ternaryalloy and a related art quaternary alloy further including anotherelement such as Ta.

Niobium (Nb), which is a major alloying element in the titanium alloy inaccordance with the exemplary embodiment, is a soft, grey, ductilemetal. Niobium is known as a biocompatible metal because it is stableand does not undergo toxic reactions with fiber cells, corrosionproducts, and bio-solutions in the human body. In addition, niobium isvery stable at room temperature, and has very high corrosion resistanceso that it is not corroded by oxygen and strong acids. It is preferablethat niobium is contained in the titanium alloy in a weight percentageranging from 37 wt. % to 41 wt. %. This is because the beta phase isdifficult to form sufficiently outside this composition range, and thusthe elastic modulus increases considerably to 70 GPa or higher. It ismore preferable that niobium is contained in the titanium alloy in aweight percentage ranging from 38 wt. % to 40 wt. %.

Zirconium (Zr) has very high corrosion resistance in hot water underacidic or basic atmosphere. Zirconium forms oxide film even in air,showing high corrosion resistance. Zirconium is a biocompatible metalwithout the cytotoxic effect. It is preferable that zirconium iscontained in the titanium alloy in a range from 5 wt. % to 8 wt. %. Thisis because the elastic modulus of the ternary alloy of titanium, niobiumand zirconium increases considerably outside this range, so that itcannot be applied to a living body. It is more preferable that zirconiumis contained in the titanium alloy in a range from 5 wt. % to 7 wt. %.

According to an exemplary embodiment, it is possible to lower theelastic modulus of the titanium alloy to 50 GPa or lower, as well as to55 GPa or lower.

According to use, one or more elements selected from tantalum (Ta),hafnium (Hf), molybdenum (Mo), and tin (Sn) may be further added in thetitanium alloy in a range of 3 wt. % or lower. It is preferable thatthey are added in a range from 1 wt. % to 3 wt. % in view of the elasticmodulus factor. In this case, it is preferable that the content ofniobium is from 37 wt. % to 39 wt. %, and the content of zirconium isfrom 5 wt. % to 7 wt. %.

Tantalum (Ta) is ductile, and has high mechanical strength even at hightemperature. Tantalum forms a stable film with high electric resistanceso that it is relatively free from oxidation in air. In addition,tantalum is highly resistant to acid, and has excellent compatibilitywith the human body, so that it can be used for cementing bones.Tantalum, when alloyed in titanium, serves as a major beta stabilizer.

Hafnium (Hf) has characteristics very similar to zirconium, and hasexcellent corrosion-resistance and bio-compatibility. It serves as abeta stabilizer when alloyed in titanium.

Molybdenum (Mo) has a relatively high melting point. However, it hasexcellent thermal conductivity, high corrosion resistance even in strongacid, and very favorable mechanical properties over a wide temperaturerange. It serves as a beta stabilizer when alloyed in titanium.

Tin (Sn) is stable in an air and has excellent ductility. It is solublein acids and alkalis, and has a very low melting temperature of about232° C. It is stable in the human body and thus widely used in thefields of table ware, plating and the like. It may also serve as a betastabilizer when alloyed in titanium.

Addition of the above elements in an amount greater than 3 wt. % mayaffect the titanium-niobium-zirconium ternary system to increase theelastic modulus. Accordingly, the maximum content of the above-mentionedelements in the titanium alloy is set to 3 wt. % or lower.

The titanium alloy in accordance with the exemplary embodiments can befabricated by various melting or casting methods such as vacuuminduction melting (VIM), vacuum arc remelting (VAR), induction skullmelting (ISM), plasma arc melting (PAM), electron beam melting (EBM) andthe like.

ADVANTAGEOUS EFFECTS

The beta-based titanium alloy in accordance with the exemplaryembodiments of the present invention has low elastic modulus andexcellent mechanical properties. Therefore, it can be used in a varietyof applications, for example, as a material for medical devices, such asartificial bones, artificial teeth and artificial hip joints, as amaterial for general civilian goods such as eyewear frames and headsets,and as a material for sports and leisure goods such as golf clubs.

DESCRIPTION OF DRAWINGS

FIG. 1 is a photograph of an ingot prepared by melting and casting atitanium alloy in accordance with an exemplary embodiment.

FIG. 2 is a photograph of a cylinder-shaped product prepared by drawingthe ingot of FIG. 1.

FIGS. 3 and 4 are micrographs, each showing a microstructure of atitanium alloy in accordance with Embodiment 1.

FIG. 5 is a micrograph showing a microstructure of a titanium alloy inaccordance with Embodiment 2.

BEST MODE

Hereinafter, specific embodiments will be described in detail withreference to the accompanying drawings. However, it should be understoodthat the description of the embodiment is merely illustrative and shouldnot be taken in a limiting sense.

Embodiment 1

Ti—Nb—Zr ternary alloys having compositions as listed in Table 1 wereprepared by a vacuum arc remelting (VAR) process.

In order for uniform alloy composition, process convenience, processeconomy, time and energy savings and the like, a Ti—Nb master alloy wasused to cast beta-based titanium alloys.

The titanium alloys melted by the VAR process in accordance with theembodiment were cast into ingots as shown in FIG. 1. Then, the ingotswere processed into bars having a diameter of 15 mm as shown in FIG. 2,through a drawing process.

The ingot had an excellent appearance. Surface crack, fracture and thelike that are often generated during the drawing process were notobserved in the surface of the bar. Accordingly, it can be concludedthat the titanium alloys in accordance with the embodiment have goodformability and good workability.

The alloy bar fabricated in accordance with the embodiment was cut intoa section perpendicular to the drawing direction and a section parallelto the drawing direction. The cut surface was first macro-polished withabrasive papers of up to 2400 grit and then micro-polished with adiamond paste.

After the mechanical polishing, the cut surface was etched with Krolletchant (H₂O 100 ml+HNO₃ 5 ml+HF 3 ml) and then the microstructure ofthe cut surface was observed using an optical microscope.

FIG. 3 is a micrograph (at 200× magnification) of a surface of thespecimen No. 4 (Table 1) cut perpendicular to the drawing direction.FIG. 4 is a micrograph (at 200× magnification) of a surface of thespecimen No. 4 (Table 1) cut parallel to the drawing direction.Referring to FIGS. 3 and 4, the beta-based titanium alloy fabricated inaccordance with the embodiment had uniform grain size, and showed nosegregations and no defects.

Four test specimens were taken from the titanium alloy in accordancewith the embodiment. Then, an elastic compression test was performedaccording to ASTM E9-89a specifications. The average elastic moduli ofthe specimens obtained from the elastic compression test are given inTable 1.

TABLE 1 Composition of alloy Elastic modulus Specimen No. (wt. %) (GPa)Remarks 1 Ti—34Nb—11Zr 68 Comparative 2 Ti—35Nb—8.2Zr 72 Comparative 3Ti—37.9Nb—7.4Zr 41.5 Experimental 4 Ti—38.9Nb—5.5Zr 38.9 Experimental 5Ti—39Nb—6Zr 40 Experimental 6 Ti—40.9Nb—5Zr 40 Experimental 7Ti—42.4Nb—5.5Zr 74 Comparative 8 Ti—43Nb—12Zr 81 Comparative

As can be seen from the measured elastic modulus data in Table 1,contents of niobium and zirconium in the comparative examples weresimilar to those in the experimental examples. However, the elasticmoduli of the comparative examples were 80% to 100% greater than thoseof the experimental examples.

That is, by minimizing the addition of the alloying elements through anew alloy design for restricting the amount of the alloying elements,the ternary titanium alloy in accordance with the embodiment can achievethe ultra-low elastic modulus, which has been difficult to achieve evenin related art quaternary titanium alloys.

MODE FOR INVENTION

Contrary to Embodiment 1, a titanium alloy in accordance with Embodiment2 further includes tantalum (Ta) as shown in Table 2, so as to improvemechanical properties while still maintaining the low elastic modulusand including no elements harmful to the human body. The titanium alloyswere melted by the vacuum arc remelting (VAR) process, cast into ingots,and then drawn into bars, as described in Embodiment 1.

Specimens were cut from the alloy bars and polished mechanically. Afteretching the specimen, the microstructure was observed at a magnificationof 50× using an optical microscope. As shown in FIG. 5, there were nosegregations and no defects visible in the microstructure of the alloy.

Further, the elastic compression test was performed four times accordingto ASTM E9-89a specifications to measure the elastic modulus of thealloy. The average elastic moduli of the specimens obtained from theelastic compression test are given in Table 2.

TABLE 2 Specimen Composition of alloy Elastic No. (wt. %) modulus (GPa)Remarks 9 Ti—37.3Nb—5.8Zr—2.9Ta 43 Experimental 10 Ti—39Nb—6.5Zr—1.5Ta39 Experimental

As shown in Table 2, the titanium alloys in accordance with Embodiment 2were not increased in the elastic modulus in comparison with thetitanium alloys in accordance with Embodiment 1. Accordingly, thetitanium alloy in accordance with Embodiment 2 can be used to achievethe required mechanical properties as well as the elastic modulus.

1. A titanium alloy comprising 37 wt. % to 41 wt. % niobium (Nb), 5 wt.% to 8 wt. % zirconium (Zr), and a balance of titanium (Ti), withunavoidable impurities, and having an elastic modulus of 55 GPa orlower.
 2. The titanium alloy of claim 1, wherein the elastic modulusthereof is 50 GPa or lower.
 3. The titanium alloy with a low elasticmodulus of claim 1, further comprising one or more elements selectedfrom tantalum (Ta), hafnium (Hf), molybdenum (Mo) and tin (Sn), whosetotal content is 3 wt. % or lower.
 4. The titanium alloy with a lowelastic modulus of claim 1, wherein the content of niobium (Nb) is 38 wt% to 40 wt. %, and the content of zirconium (Zr) is 5 wt. % to 7 wt. %.5. The titanium alloy with a low elastic modulus of claim 2, wherein thecontent of niobium (Nb) is 37 wt. % to 39 wt. %, the content ofzirconium (Zr) is 5 wt. % to 7 wt. %, and the titanium alloy furthercomprises 1 wt. % to 3 wt. % tantalum (Ta).
 6. The titanium alloy with alow elastic modulus of claim 2, wherein the content of niobium (Nb) is38 wt. % to 40 wt. %, and the content of zirconium (Zr) is 5 wt. % to 7wt. %.